Headers are expensive for the early a-body car, plus, I don't think you can run power steering with ANY of the headers available for the 63-66 car. If you hav a 318, you are better off running the original 273 manifolds.
You can add power brakes but you may need to get the original parts like the booster mounting bracket and booster, they are also early-a only parts...but if interested PM me, I can possibly get the booster and bracket from a 66 Valiant. What front brakes are you currently running? SBP discs are getting hard to find, you can also use 73-76 A-body discs, easy swap but then you will have the BBP up front. There is another way too, get the Scarebird kit which uses your old spindles and adapts newer parts that you can get cheap and any parts store, and you can still maintain the SBP up front!
No sure on the door panels...1966 Valiant has alot of 1-year only parts, the trunk lid and doors are the only parts interchangeable. The 66 Cuda shares some parts that are the same too. Window seals are available, you can get new chrome plastic locking strips on eBay for the rubber seals too.
Any more questions let us know...I am the 66 Valiant guy!! :glasses7: